1. Field of the Invention
The present invention relates to a single phase stepping motor having a rotor that is disposed so as to surround a stator.
2. Related Art
Stepping motors are used in various fields because they are simple in structure and easy to control. A permanent magnet (PM) stepping motor, in particular, is widely used in various fields because it can be manufactured economically.
With this type of stepping motor, reliability is important, and it is required that the motor starts up surely normally under conditions of all kinds. To ensure normal startup, it is necessary to control the stepping motor so that it stops at predetermined precise positions. In order to control the stop position at high precision, a detent torque is generally adjusted as small as possible. This is because the detent torque functions as a load when the rotor rotates. Here, the “detent torque” means a torque held when a power is not supplied. On the other hand, a torque held when the power is supplied is called as a holding torque.
However, where the load is large, for example like a stepping motor used for generating vibration, when the load is an eccentric weight, it is difficult to control the rotor to surely stop at desired stop positions when the power is not supplied. Accordingly, there is proposed a technique in which the detent torque is increased to surely stop the load when the power is not supplied (JP-A (Kokai) No. 60-43059, JP-A (Kokai) No. 6-78513 and JP-A (Kokai) No. 9-308214).
As one type of PM stepping motor, there is an outer rotor type single phase stepping motor, in which the rotor is disposed so as to surround an annular stator. The stator has stator yokes in which a plurality of pole teeth are formed, and the rotor has a magnet disposed around a circumference thereof. The magnet has a plurality of magnetic poles of which neighboring magnetic poles have different polarities.
In the outer rotor type of single phase stepping motor, the stable positions (stop positions) of the holding torque are positions where the centers of the magnetic poles of the magnet approximately coincide with the centers in width dimensions of the pole teeth of the stator yoke. As many positions exist as the total number of the magnetic poles of the magnet with respect to each direction of the current flowing through the stator coil. Thus, if the total number of the magnetic poles of the magnet is eight, there will be four stable positions (stop positions) for each of the current directions. For example, assuming that each of the magnetic poles has equal width, stable positions of the holding torque is provided at 90-degree intervals in each current direction flowing through the stator coil. On the other hand, the stable positions (stop positions) of the detent torque are also the positions where the centers of the magnetic poles of the magnet approximately coincide with the centers in width dimensions of the pole teeth of the stator yoke. As many positions exist as the total number of the magnetic poles of the magnet.
In this way, when the stable positions of the holding torque coincide with the stable positions of the detent torque, there is a concern that the startup after a stop may not be normally performed. A solution for this concern is to provide commutating poles that increase/decrease the width of some of a plurality of pole teeth of the stator yoke. The reason for providing such commutating poles is that if the commutating poles are not provided and the single phase stepping motor is tried to start up at the stable position, the motor might fail to start up normally, or might operate only a half of one step. If the commutating poles are provided, it is possible to shift the stable positions of the holding torque from the stable positions of the detent torque to each other, thereby stabilizing startup.
The commutating poles in the single phase stepping motor are provided in some of a plurality of pole teeth in each of a pair of stator yokes located on both sides of the stator coil. The commutating poles alter the phase between the pole teeth of the two stator yokes, thereby improving the startup performance of the motor.
However, even if the phase is adjusted by the commutating poles, the stable positions of the holding torque or the detent torque vary depending on the positions and sizes of the commutating poles. The stable positions of the holding torque or detent torque may also vary due to non-uniformities and other factors occurring during manufacture and assembly.
Further, even if commutating poles are provided, what are called “dead zones”, where the stable positions of the holding torque or detent torque spread out, may arise due to friction loss or other factors. Should the width of the dead zones become large, the stable positions of the holding torque will easily coincide with the stable positions of the detent torque, resulting in poor startup performance of the motor.